The present invention relates to an electrolyte for non-aqueous electrolyte electrochemical device and a non-aqueous electrolyte electrochemical device by the use thereof. More particularly, it relates to a non-aqueous electrolyte lithium secondary battery.
Non-aqueous electrolyte electrochemical devices containing a negative active material comprising a light metal such as lithium, sodium and the like have found the application in the wide areas of various electrical and electronic appliances. The non-aqueous electrolyte electrochemical devices include batteries, capacitors for electronic double layer and the like. Particularly, the non-aqueous electrolyte secondary batteries are charge-discharge batteries having a high energy density and capable of miniaturization and lightening and have now been researched and developed on an extensive scale.
A non-aqueous electrolyte secondary battery is constituted with a positive electrode, a negative electrode and a separator (diaphragm) keeping the positive electrode apart from the negative electrode.
The solvents used in the electrolytes for non-aqueous electrolyte battery are mostly cyclic carbonates represented by propylene carbonate (PC) and ethylene carbonate (EC); chain carbonates represented by diethyl carbonate (DEC) and dimethyl carbonate (DMC); cyclic carboxylates represented by xcex3-butyrolactone (GBL) and xcex3-valerolactone (GVL); chain ethers such as dimethoxymethane (DMM), 1,3-dimethoxypropane (DMP) and the like; and cyclic ethers such as tetrahydrofuran (THF) or 1,3-dioxolane (DOL) and the like.
When these solvents are applied to the non-aqueous electrolyte secondary batteries, those having a high electrical conductivity or a high relative permittivity but a low viscosity are preferred. However, the high relative permittivity means nothing but a strong polarity that is accompanied by a high viscosity. In many cases of the practical batteries now available, therefore, the solvents having a high permittivity such as ethylene carbonate (permittivity xcex5=90) and the solvents having a low permittivity such as dimethyl carbonate (DME, xcex5=3.1) or ethylmethyl carbonate (EMC, xcex5=2.9) are used in combination, chosen from among the electrolytes above.
The electrolytes used in non-aqueous electrolyte batteries are prepared by dissolving supporting electrolytes in said solvents in a concentration of approximately 1 mol. The supporting electrolytes which can be used herein are anion lithium salts of inorganic acid represented by lithium perchlorate (LiClO4), lithium borofluoride (LiBF4) and lithium phosphofluoride (LiPF6); and anion lithium salts of organic acid such as lithium trifluoromethanesulfonate (LiSO3CF3), imidolithium bistrifluoromethanesulfonyl imide lithium ((CF3SO2)2NLi) and the like.
The separators which can be used herein are insoluble in said non-aqueous electrolytes, for example, comprising a polyethylene or polypropylene resin porous diaphragm.
The positive active materials which can be used herein are lithium of cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4, LiMnO2) and lithium ferrate (LiFeO2); parts of these transition metals (Co, Ni, Mn, Fe) substituted with other transition metals such as tin (Sn), aluminum (Al) and the like; transition metal oxides such as vanadium oxide (V2O5), manganese dioxide (MnO2), molybdenum oxide (MoO2, MoO3) and the like; and transition metal sulfides such as titanium sulfide (TiS2), molybdenum sulfide (MoS2, MoS3), iron sulfide (FeS2) and the like.
The negative active materials which can be used herein are lithium ions or sodium ions, and the negative host materials useful for them are amorphous carbon materials; carbon materials such as artificial or natural graphite calcined at 2000xc2x0 C. or higher; metals such as aluminum (Al), lead (Pb), tin (Sn), bismuth (Bi), silicon (Si) and the like, capable of alloying with alkaline metals; interstitial alkaline metal inserted type crystal system based intermetallc compounds (AlSb, Mg2Si, NiSi2); and lithium nitrogen compounds (Li(3xe2x88x92x)MxN (M: transition metal)) and the like.
In recent years, the non-aqueous electrolyte secondary batteries using the above host materials capable of storing and releasing the alkaline metal ions in the negative electrodes have been predominant, replacing the other non-aqueous electrolyte secondary batteries with alkaline metals performing the double role of active material source and electronic collector.
Since the non-aqueous electrolyte secondary batteries have characteristics of high voltage and high energy density, the negative host materials used for them are preferably the substances generating a voltage close to that of alkaline metals, i.e. above amorphous carbon materials or carbon materials such as artificial or natural graphite calcined at 2000xc2x0 C. or higher. In these substances, however, the alkaline metal ions contained therein are liable to react with the electrolytes at high temperatures, giving rise to the evolution of heat or gas.
On the other hand, the non-aqueous electrolyte secondary batteries are high in voltage and also high in the energy density, and it is likely that the oxidative decomposition of the solvents or solutes would occur even on the positive electrodes. The higher the temperatures are the more conspicuous these phenomena are, and when the batteries are stored at such high a temperature as 60xc2x0 C. or 85xc2x0 C., the reductive decomposition occurs at the side of negative electrodes and the oxidative decomposition at the side of positive electrodes with the result that a great deal of gas evolves. Furthermore, in recent years the non-aqueous electrolyte secondary batteries are widely used as the backup power source in note type personal computers, the temperatures are always from 45xc2x0 C. to 60xc2x0 C. in the inside of the note type personal computers, constant voltage of 4.2V is applied to enable the computers to maintain the high capacity at such high temperatures all the times, and thus gas is easy to evolve therein.
If gas evolves at the time of high temperatures, pressure rises within the batteries to drive the safety devices, cutting off the electric current or causing the deterioration of battery characteristics, and there has been a strong demand for improvement.
In the high temperature environment as described above, catalyst power to oxidize is made stronger particularly on the positive electrodes and the oxidative decomposition of the non-aqueous solvents occurs on the surface of positive electrodes, decreasing the conductivity of electrolyte to deteriorate discharge characteristics or developing the decomposition product in the form of gas (for example, carbon dioxide gas) and, if the worst happens, the electrolytes have been found to leak.
In an attempt to find a solution in said problems, many methods have been proposed for incorporating the additives capable of forming films on the positive and negative electrodes into the electrolytes. Although these additives are effective in inhibiting gas from evolving, however, they have a problem that many of them are inclined to form highly resistant films on the electrodes, deteriorating the batteries in their characteristics of charge and discharge, particularly characteristics of high rate discharge or those of low temperature discharge.
In Hyomen Gijutsu (The Journal of the Surface Finishing Society of Japan), 50 (5), 460 (1999), Kunio Mori et. al. have disclosed a technique for forming an organic deposit film from a 6-substituted-1,3,5-triazine-2,4-dithiol. It has been described that the organic deposit films can find an application in imparting mold release characteristics to metal molds or in directly bonding metals with polymers or in techniques for preventing metals from corroding away.
In a process of investigating the additives in the electrolytes, the present inventors have found that an organic film can be formed on the positive electrode in full control of film forming voltage and rate under the ordinary manufacturing and/or use conditions of batteries by incorporating at least one of 6-substituted-1,3,5-triazine-2,4-dithiol and its derivatives into a non-aqueous electrolyte of a non-aqueous electrolyte electrochemical device comprising at least two electrodes, wherein the non-aqueous electrolyte comprises a solute dissolved in a non-aqueous solvent and a separator arranged between the two electrodes. These organic films have been found to be stable and capable of preventing the non-aqueous electrolytes from coming into the direct contact with the electrodes and consequently taking an effect of inhibiting the deterioration of characteristics attributed to gas that would be evolved at the time of charging or high temperature storage. Furthermore, it has also been found that the so obtained films are low in resistance, making it possible to provide a novel non-aqueous electrolyte electrochemical device useful in a wide range of temperatures, having a high energy density, low in the decrease of the discharge capacity even after the repeated use and excellent in high rate charge-discharge characteristics, and the present invention has been completed on the basis of these findings.
The present invention relates to an electrolyte for non-aqueous electrochemical device, which comprises at least one of 6-substituted-1,3,5-triazine-2,4-dithiol and derivatives thereof represented by the formula (I): 
wherein R is a nitrogen-containing functional group; and M is a hydrogen or an alkali metal.
The present invention provides an excellent novel non-aqueous electrochemical device useful in a wide range of temperatures by using the electrolyte in a non-aqueous electrochemical device comprising at least two electrodes, a non-aqueous electrolyte containing a solute dissolved in a non-aqueous solvent and a separator arranged between the two electrodes.
Furthermore, the present invention provides an novel lithium secondary battery having a high energy density, low in the decrease of the discharge capacity attributable to the repeated use of the battery and excellent in high rate charge-discharge characteristics by using the non-aqueous electrolyte in a lithium secondary battery comprising a positive electrode, a negative electrode containing a lithium as the active material, an electrolyte containing a solute having a lithium as a cation that is dissolved in a non-aqueous solvent and a separator arranged between the two electrodes.
The compound 6-substituted-1,3,5-triazine-2,4-dithiol and derivatives thereof used in the present invention are represented by the following formula (I): 
wherein R is a nitrogen-containing functional group; and M is a hydrogen or an alkali metal. The nitrogen-containing functional group is preferably represented by the formula xe2x80x94NR1R2. In the formula, R1 and R2 are each independently alkyl, aryl or hydrogen, preferably alkyl.
The term alkyl used herein means straight-chain C1-30 alkyl or branched chain C1-30 alkyl and branched or unbranched C3-30 cycloalkyl unless otherwise described. These groups may as well be either saturated or unsaturated, unless otherwise described.
Particularly, saturated or unsaturated C1-10 alkyl, for example, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, or vinyl, allyl, isopropenyl and the like are preferable.
The term aryl used herein means a group containing the aromatic rings singly or in combination of two or more, and the specific examples of ring include, but not limiting to these examples, benzene, biphenyl, terphenyl and the like. These rings may as well be optionally substituted with alkyl above, particularly preferably C6-30 aryl.
The alkaline metal represented by M is preferably sodium, potassium, lithium, calcium or magnesium, more preferably sodium or lithium and most preferably lithium.
The preferable specific examples of the compound represented by the formula (I) are 6-dimethylamino-1,3,5-triazine-2,4-dithiol, 6-diethylamino-1,3,5-triazine-2,4-dithiol, 6-dipropyloctylamino-1,3,5-triazine-2,4-dithiol, 6-dibutylamino-1,3,5-triazine-2,4-dithiol, 6-dipentyloctylamino-1,3,5-triazine-2,4-dithiol, 6-dihexylamino-1,3,5-triazine-2,4-dithiol, 6-diheptylamino-1,3,5-triazine-2,4-dithiol, 6-dioctylamino-1,3,5-triazine-2,4-dithiol, 6-dinonylamino-1,3,5-triazine-2,4-dithiol, 6-didecylamino-1,3,5-triazine-2,4-dithiol, 6-divinylamino-1,3,5-triazine-2,4-dithiol, 6-diallylamino-1,3,5-triazine-2,4-dithiol and 6-diisopropenylamino-1,3,5-triazine-2,4-dithiol, and their sodium, potassium, lithium, calcium or magnesium salt.
The more preferable specific examples are the 6-dioctylamino-1,3,5-triazine-2,4-dithiol and 6-diallylamino-1,3,5-triazine-2,4-dithiol and their sodium or lithium salt.
The amount of addition varies with the compounds of the formula (I) but these compounds are added generally in an amount of 1xcx9c100 mM.